The present invention generally relates to the art of floating roof storage tanks, and more particularly to a device for reducing the release of vapors to atmosphere through the open slots of a gauge well in a floating roof tank.
Floating roof tanks are in widespread use for the storage of volatile stock materials, particularly petroleum products. In order to enable the level of the material in a tank to be measured, the tank is provided with a gauge well that generally comprises a vertical tube that is disposed within the tank adjacent its shell, i.e. side wall. The gauge well is fixed in position within the tank, and as the floating roof of the tank moves up and down with the level of the stock stored therein, it slides along the gauge well.
In order to permit accurate sampling of the stock within the tank, the gauge well is provided with slots spaced along its length. It may be desirable to obtain samples of the stock at various levels within the tank, for example prior to sale of the contents of the tank. A reading of the stock at these levels can be obtained with a sampling device that is lowered into the gauge well to the desired levels. If the slots were not provided along its length, then some other means of sampling would be required since the liquid within the gauge well would only be representative of that at the bottom of the tank where it entered the gauge well.
Since the gauge well, and more particularly the slots therein, provide an opening to atmosphere through which vapors from the liquid stock can escape, it is desirable, and in fact required under some environmental control restrictions, to restrict these vaporous emissions. This has typically been done by means of a float disposed within the gauge well. The float rides up and down within the gauge well and effectively limits the surface area of the liquid within the gauge well from which evaporation can occur.
While the gauge well float is effective in reducing evaporation from floating roof tanks, the designs of those used heretofore have presented some practical difficulties. To facilitate removal of the float from the gauge well when a sample of the stock is to be taken, a chain is attached at one end to the float and at the other end to the top of the gauge well, for example its hatch. The length of the chain is at least equal to the depth of the gauge well. When the float is disposed near the middle of the gauge well, for example, it will be appreciated that there is substantial slack in the chain. Sometimes the chain can get caught on burrs that are formed when the slots are cut into the well. More typically, however, the slack portion of the chain gets blown outside of the gauge well slots by the wind, where it kinks and gets caught. In either case, when the chain gets hung up in this manner it can result in the float being suspended above the level of the liquid stock. Alternatively, when the chain is caught it can hold the float down, causing it to be submerged when the tank is filled. In both of these situations the purpose of the float is defeated.
In the past, the gauge well float comprised a hollow cylindrical cannister constructed of metal components, typically bronze, that were welded, brazed or soldered together. Since many of the liquid materials that are commonly stored in floating roof tanks have a corrosive effect on the metal float, the float can develop leaks after a period of contact with the stock. More particularly, the joints between the components can crack or become corroded. When a float with cracks or the like is submerged as described above, it fills with the liquid and no longer becomes an effective deterrent to evaporation, even if the chain is subsequently freed.
Another problem associated with prior types of gauge well floats is presented by the burrs that are on the inside wall of the gauge well. Specifically, the edges of the float often get caught on them, resulting in the float being hung up within the gauge well above the level of the stock. Consequently, if there are any gauge well slots present between the float and top of the liquid, evaporation will not be prevented or reduced.